1. Field of the Invention
The present invention relates to a method of fabricating a package for housing a semiconductor element.
2. Related Art
In the past, a package for housing a semiconductor element such as LSI (large-scale integrated circuit), which is generally made of electrical insulating materials like alumina ceramic and comprises an insulating substrate having a depression at nearly the center portion on top of the package, a plurality of wiring layers formed of a metal such as tungsten or molybdenum which are led away from the periphery of the depression of the insulating substrate and extending to an underside thereof, a plurality of electrical connection pads which are formed on the underside of the insulating substrate and are electrically connected to the wiring layers, connection ball terminals consisting of solder hard-soldered to the connection pads, and a lid, become a semiconductor device that can be fabricated as a product by securely bonding the semiconductor element within the depression of the insulating substrate using an adhesive consisting of a material such as glass or resin; electrically connecting the respective electrodes of the semiconductor element to the wiring layers by means of bonding wires while joining the lid to the upper surface of the insulating substrate using a sealant such as glass or resin; and hermetically sealing the semiconductor element inside a container consisting of the insulating substrate and the lid.
Such semiconductor devices are mounted on an external electrical circuit board by installing ball connection terminals, which are formed of solder hard-soldered to the electrical connection pads on the underside of the insulating substrate, on the wiring conductors of the external electrical circuit board; thereafter fusing the connection terminals at a temperature of from about 200 to about 300.degree. C. to join the connection terminals to the wire conductors. Simultaneously, the respective electrodes of the semiconductor element housed inside the package for housing a semiconductor element are electrically connected to the external electrical circuit by means of the wiring layers and the connection terminals.
The prior art includes some known methods of connecting such connection ball terminals to wiring boards such as external electric circuits. For example, the method disclosed in U.S. Pat. No. 3,719,981 (issued to Steitz) is designed so that solder balls are joined to low-profile solder bumps forming part of the primary layer on a substrate in order to allow electrical connections to a printed circuit board. A tacky pressure-sensitive tape is used for maintaing an alignment of the solder balls in a row while the solder balls are properly aligned with the solder bumps and reflowed to form connection ball terminals. This method, however, is considered lengthy and involves too many steps. In addition, control over the ultimate size of the reflowed solder balls (that is, the size of the connection ball terminals) is considered to be inadequate.
Another method is disclosed in U.S. Pat. No. 5,203,075 (issued to Angulas, et al.), in which the formation of interconnections between circuit substrates is achieved by fusing solder paste deposits applied to pads on either of the substrates. More specifically, the solder paste melts and balls up around a solder ball attached to a substrate opposite to the substrate with the solder paste applied thereto to form spherical lumps (connection ball terminals). When this method is used, however, some problems occur when the solder balls are attached to the opposite substrate in advance.
When the solder ball is mounted in a depression on the substrate, the solder ball does not greatly protrude from the underside of the substrate in cases where the diameter of the solder balls is smaller than the opening (diameter) of the depressions, and this makes it difficult to connect the solder balls to another circuit substrate. On the other hand, in cases where the diameter of the solder balls is greater than the opening (diameter) of the depressions, the solder balls are brought into contact with the opening of the depressions and prevented from contact with connection pads in the depressions on the counterpart substrate, and as a result, the solder balls cannot be securely connected to the connection pads.
U.S. Pat. No. 5,133,495 (issued to Angulas et al.) discloses yet another method, in which deposits of solder paste surrounded by dewetting material are heated so that the solder paste forms connection ball terminals which are electrically connected to the connection pads beneath the solder deposits. The electrical connections between the substrates are established by connecting the connection ball terminals to the connection pads provided in the depressions on the twinned substrate. According to this method, however, it is difficult to make terminals of a uniform size (particularly, diameter), since the connection ball terminals are formed of only solder paste, without using solder balls. If the sizes of the connection ball terminals are not properly controlled, precisely the same problem occurs as that related with the size correlation between the solder balls and the depressions on the circuit substrate, described above concerning the disclosure of U.S. Pat. No. 5,203,075.
Yet another method is disclosed in U.S. Pat. No. 5,442,852 (issued to Danner), in which solder balls placed in depressions provided on a substrate are fused and connected to connection pads in the depressions to form connection ball terminals together. The connection ball terminals may be used to establish electrical connections between a separate wiring board such as an external electric circuit and the substrate provided with the connection ball terminals.
When this method is followed, however, a variety of problems such as those mentioned above arise, since only solder balls are fused to form connection ball terminals, without using solder paste. More specifically, when the solder balls are mounted in the depressions, the solder balls do not greatly protrude from the underside of the substrate in cases where the diameter of the solder balls is smaller than the opening of the depressions, and this makes it difficult to connect the connection ball terminals to the external electric circuit.
On the other hand, in cases where the diameter of the solder balls is greater than the opening of the depressions, the solder balls are brought into contact with the opening of the depressions and prevented from contact with the connection pads, and as a result, the solder balls cannot be securely connected to the connection pads. In order to solve the problem, a reasonable suggestion is to reduce the depth of the depressions in order to allow the solder balls to be brought into partial contact with the connection pads according to the prior art method. Nevertheless, the following problems are brought about when ceramic such as alumina ceramic is used to form the insulating substrates. Specifically, in those packages for housing semiconductor elements according to the prior art, the coefficient of thermal expansion of the insulating substrate made of alumina ceramic or the like is 6.5.times.10.sup.-6 or greater, whereas an external electrical circuit board, which is generally made of glass epoxy, has a coefficient of thermal expansion of from 2.times.10.sup.-5 /.degree.C. to 4.times.10.sup.-5 /.degree.C. Because there is a large difference between the two coefficients, when the semiconductor element is housed inside the package for housing a semiconductor element to fabricate a semiconductor device which is then mounted on the external electrical circuit board and, heat generated while the semiconductor element is operated is repeatedly applied to both the insulating substrate and the external electrical circuit board, a large thermal stress occurs between the insulating substrate of the package and the external electrical circuit board due to the difference of both coefficients of thermal expansion. This stress acts on the peripheral portion of the connection pads on the underside of the insulating substrate to separate the connection pads from the insulating substrate. A defect which occurs as a result of this is that, over a long period of time, the respective electrodes of the semiconductor element housed inside the package for housing a semiconductor element can not be electrically connected to predetermined external electrical circuits.